The IP Multimedia Subsystem (hereinafter IMS) enables operators of a Public Land Mobile Network (hereinafter PLMN) to provide their subscribers with multimedia services based and built on Internet applications, services and protocols. Different services and applications as defined by the 3rd Generation Partnership Project (hereinafter 3GPP) can be offered on top of IMS, for example a Multimedia Telephony Service as described in 3GPP TS 22.173.
An IMS application, as well as IMS-provided services, includes a exchange of media, such as voice or video, and of signalling for a right interpretation and treatment of the media. In a scenario where a user accesses the IMS through an access network based on an Internet Protocol (hereinafter IP), both signalling and media are transported through a bearer layer as respective signalling and media IP packets, which are generally and respectively referred as signalling IP flow and media IP flows.
A complete support for IMS applications includes a user equipment used by the user who is an IMS subscriber, an IP-Connectivity Access Network (hereinafter IP-CAN) and specific functional elements of the IMS as described in 3GPP TS 23.228. An exemplary IP-CAN may be a General Packet Radio Service (hereinafter GPRS) network.
Before being authorized to make use of IMS services provided by IMS applications, the user has to register into the IMS. However, before being able to register into the IMS, the user has to get IP connectivity to IP-CAN. To this end, and in case the IP-CAN is a GPRS network, the user has to establish a bearer for transporting signalling IP flows, namely the so-called “PDP Context”.
Even though IMS signalling and further media flows may be both transported in a same general purpose PDP Context, a general purpose PDP Context is preferably used as a primary dedicated PDP Context for IMS signalling, whereas media flows are preferably transported over a separate secondary dedicated PDP Context.
Therefore, the user accesses a Gateway GPRS Support Node (hereinafter GGSN) to establish a primary PDP Context of a general purpose or, preferably, for IMS signalling only.
Once this primary PDP Context has been established to carry out the IMS signalling IP flow, the user can register into the IMS. The 3GPP IMS makes use of a Session Initiation Protocol (hereinafter SIP) for communication between the user and IMS entities as well as between the IMS entities. The user thus makes use of a SIP Register message for registering into the IMS. Such SIP Register is received at a Proxy Call Session Control Function (hereinafter P-CSCF) server, which is an IMS entity used for accessing the IMS.
In accordance with a well-known 3GPP IMS registration procedure, after having received and processed such message, the P-CSCF forwards it towards an Interrogating Call Session Control Function (hereinafter I-CSCF) server. The latter asks a Home Subscriber Server (hereinafter HSS) for capabilities required for assigning a Serving Call Session Control Function (hereinafter S-CSCF) server for servicing the user. Once the I-CSCF has received such capabilities selects and assigns one S-CSCF fulfilling these capabilities, and the original SIP Register message is forwarded towards the assigned S-CSCF. This S-CSCF requests from the HSS a user profile for the user and, once obtained, the S-CSCF is ready for servicing the user. An IMS registration confirmation is sent back towards the P-CSCF and towards the user indicating the assigned S-CSCF.
Once the user has successfully registered into the IMS, the user can access different IMS applications, and request different IMS services, for which different media resources have to be negotiated between the user equipment of the originating user and either a specific IMS application or a user equipment of a destination user, as the IMS service might require. To this end, the user accesses to IMS with a SIP Invite message, which is received at the P-CSCF and transmitted through the IMS infrastructure towards the S-CSCF and likely further towards an Application Server (hereinafter AS) in charge of a particular IMS application that the user might have invoked. This message has an associated media description required for carrying out the service. This media description is sent with a Session Data Protocol (hereinafter SDP) and is used during the negotiation of the media resources required for carrying out the service.
All the above IMS signalling, SIP signalling during the IMS registration procedure as well as both SIP and SDP signalling during media negotiation procedure, are thus transported as an IMS signalling IP flow in the primary PDP Context established by the user with the GGSN.
During the execution of these IMS registration and media negotiation procedures there is no media to be exchanged yet through the IP-CAN and, consequently, there is no IMS media IP flow through the bearer layer yet.